Helene M. Lampe, Yngve O. Espmark, Singing Activity and Song Pattern of the Redwing Turdus iliacus during the Breeding Season, Ornis Scandinavica (Scandinavian Journal of Ornithology), Vol. 18, No. 3 (Sep., 1987), pp. 179-185
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Lampe et al. (1987) studied this question.
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